Photo-electric tube and the method of making same



Feb. 4, 1936. J E BQER 1- AL 2,029,414

PHOTO-ELECTRIC TUBE AND THE METHOD OF MAKING SAME Filed July 1, 1933 7 (5/41 59 Mme) 8 (same (was LAVA-7Q) 9(00155/04/ Mme) INVENTORS JAN MFA/DUCK 06' 5052 #4977 C. 72"! S BY JOHAA/A/ff 0711/55 5 6 7*? I} p E Paten ed Felt. 4, 1936 GSAME Jan Hendrik de Boer, Johannes Bruynes, and

Marten Cornelia 'Teves,

Eindhoven, Netherlands, assignors to Radio Corporation of America, a corporation of Delaware Application July 1, 1933, Serial No. 678,554, In Germany July 22, 1932 11 Claims. (01. 250-275) As is well known, a photo-electric tube contains an electrode which consists partly or entirely of a light-sensitive electric material, frequently of alkali-metal, for instance, caesium. When making such a tube a quantity of alkali-metal or other light-sensitive electric substance is introduced for this purpose into the tube. As, a rule this quantity is materially more than the quantity exactly required for making said electrode.-

It has been found that the excess of this material unfavorably affects the properties of the tube. For this reason it has already'been' proposed to render this excess inoffensive by introducing a quantity of carbon into the tube, this carbon absorbing the excess of alkali-metal. However, carbon has the unfavorable property of largely absorbing almost allgases, especially oxygen, nitrogen, carbon oxide and also hydro-carbon. Due to this it is very difficult to introduce 20 carbon into the tube in a degasifled state. Under certain circumstances the gases absorbed by the carbon are liberated in the finished tube due to which the tube is rendered entirely defective in most cases.

The present invention has for its purpose to avoid the said drawbacks and to simplify the process of manufacture.

Accordingto the invention the excess of alkalimetal is bound or absorbed by one or more metals disposed outside the photo-electric electrodes,

said metals forming readily an alloy with alkalimetals at temperatures below 150 C. It has been found that the metals included in the subgroup of the second and fourth groups of the periodic system are particularly active. The subgroup of the second group includes the metals, zinc, cadmium and mercury, the sub-group of the fourth group including the metals, tin, lead and germanium. Especially the metals of the sub- 40 group of the fourth group may be advantageously used. Since germanium is too expensive for being used to a large extent, the metals tin and lead enter particularly into consideration. Both metals readily form an alloy with alkali-metals 5 at room temperature.

The advantages of the method according to the invention consist, among other things, in that the metals used can be readily completely degasified, so that there is no risk that gases are 0 set free by these metals within the tube. Furthermore, no-gases are liberated when the excess of photo-electricsubstance is consumed, so that this excess need not be removed. Moreover, this elimination generally occurs at a lower tempera- 55 ture than with the known method realized with the aid of a chemical reaction, viz. at temperatures below 150 C. At these temperatures the photo-electric electrode is not damaged. Furthermore, metals may be readily introduced as a cohesive aggregate into the tube, whereas a chem- 5 ical compound must in most cases be introduced in. powder form, thus helping an undesired diffusion of the reaction within the tube.

The active surface of the metal used for removing the excess of alkali-metal may advantageous- 1o ly be rendered very large, by forming it as a thin layer by evaporation.

The invention will be more clearly understood by reference to the accompanying drawing, in

which Fig. 1 represents one modification, and

Fig. 2, a fragmentary view of another modification.

The tube illustrated in Fig. 1 comprises a cylindrical glass wall I, to which is sealed a stem comprising a pinch 2 and a stem tube 3. The anode 4 is mounted on the pinch 2 and consists of a hairpin-shaped bent metal wire which is connected to the supply wires 5 and B. The cathode of the tube consists of a silver layer 1 coated with a silver oxide layer 8 to which is applied a very thin caesium layer 9.

The wire l0 which is sealed into the pinch has secured to it a piece of tin ll serving for binding the excess of caesium. It goes without saying that it is also possible to provide the tin in another manner in the tube. It is possible, for instance, to coat the stem tube 3, before sealing the stem to the bulb of the tube, with a thin tin layer, which may be efiected by evaporation and condensation of the tin in a vacuum.

When making the tube one may proceed as follows. A small quantity of silver is secured to the anode 4, and thereupon the latter is introduced into the tube. After the tube has been exhausted,

a heating current is passed through the anode 4, so that the silver evaporates and deposits on the wall of the tube. Part of the wall of the tube is protected in a known manner against the silver deposit by means of a screen (not shown). This shielded portion of the wall of the tube forms a window through which the light rays producing the photo-electric current may enter the tube.

After the silver layer has been provided it is superficially oxidized. For this purpose a small quantity of oxygen is introduced into the tube and an electric discharge is established between the silver layer, which serves as a cathode, and the anode. After the excess of oxygen has been removed, a quantity of caesium is introduced through the upper end of the tube. Part of the caesium is taken up by the layer of silver oxide, whereas the remainder or excess is bound by the tin II, the tin forming an alloy with the excess of caesium. This manner of eliminating the excess of caesium is extremely simple and reduces considerably the time required for making the tube.

Instead of tin it is also advantageous to use lead which, just like tin, readily forms an alloy with caesium at room temperature.

It is also possible, for instance, to use metals of the sub-group of the second group of the periodic system, viz., zinc, cadmium and mercury. When using mercury care should be taken that it is disposed in such a manner that it does not run over the cathode. For this purpose the mercury may be introduced into the tube in a special manner, for example, in form of an amalgain, for instance, a non-liquid cadmium amalgam.

According to the second modification of the invention the substance used for eliminating the excess of light-sensitive electric material, is introduced into the tube cannot combine with the light-sensitive electric material. Only after the layer, in which the particles of a chemical compound and of a photoelectric substance are contained, has been formed, is it transformed into a state, in which binding of the light-sensitive electric substance is possible. This enables one to choose at will the moment at which this substance must fulfill its task, thus avoiding the premature elimination of the excess of photo-electric material. Advantageously the substance may be introduced into the tube in such a manner that it is separated which the mixed layer is formed, the obturation being destroyed after this such an extent that it is opened.

Another manner of providing the substance in This heating member consists, for instance, of a helically wound heating wire, within which the rod-like substance is arranged. This substance has a small surface so shown in Fig. 2 wherein a metal container or capsule l2 containing tin or lead is secured to the pinch 2' of the tube by means of a supporting wire I I, a mica screen I3 being provided above this container at a short distance thereof. This lower part of the tube is placed in a high frequency magnetic field so that alternating currents are induced in the metal capsule, due to which currents the capsule partly melts and the substance contained therein is vaporized, and deposits in form of a thin layer on the wall of the tube in the vicinity of the capsule. The screen I 3 prevents the substance from penetrating into the part of the tube in which the photothis thin metal layer because tin or lead alloys with the caesium excess. The production of this alloy occurs readily at room temperature.

The substance contained in the capsule need not always consist of tin or lead. It is also possible to use other metals alloying with the lightsensitive. electric material. Furthermore, it is possible to use according to the known methods chemical compounds, for instance, lead oxide, which can react with the photo-electric material.

We claim:

1. A photo-electric tube comprising an anode containing alkalimetal, and a metal capable of alloying with the excess alkali-metal of said tube disposed outside the vicinity of said electrode.

2. A photo-electric tube comprising an anode and a photo-electric electrode containing alkalimetal, and a metal of the sub-group of the second group of the periodic system capable of al loying with the excess alkali-metal of said tube disposed outside the vicinity of said electrode.

3. A photo-electric tube comprising an anode and a photo-electric electrode with the excess alkali-metal of said tube disposed outside the vicinity of said electrode.

4. A photo-electric tube comprising an anode, a cathode containing caesium, and a metal within the tube capable of alloying with the excess caesium.

group of the second group containing zinc, cadmium 6. A photo-electric tube group of the second group of the periodic system containing zinc, cadmium and mercury.

9. A photo-electric tube comprising an anode, a light sensitive electrode containing caesium,

and a metal within the tube but at a point remote from the electrodes capable of adsorbing the excess caesium at temperatures below about 1:10 C., said metal being included in the subgroup of the fourth group of the periodic system containing tin, lead, and germanium.

10. The method of preparing the light sensitive cathode of a photo-electric tube, which consists in depositing an alkali-metal on the cathode surface, introducing within the tube a metal 'to bind said excess.

.which upon being heated forms with the excess temperatures below 150" C., and then heating said 10 metal to bind said excess.

JAN HENDRIX as BOER. JOHANNES BRUYNES. MARTEN CORNELIS 'I'EVES. 

